GPS plots a Rhumb Line or Great Circle?

[silver-fox]

Can anybody explain to me which course, great circle or rhumb line my GPS would take me on if I was to follow it to a distant destination? (and why!)

Its driven me nearly crazy trying to deduce an answer. I have concluded that it all depends on whether the earth is reperesented mathematically as a sphere or a projection but the bottom line is I simply don't have the knowledge.

 

[demonboy]

I was about to answer this with confidence.........and then I realised I don't know the answer! Surely this is a question we should all know the answer to? /forums/images/graemlins/confused.gif /forums/images/graemlins/blush.gif

 

[Jonny_H]

This was debated to death on Scuttlebutt a few weeks ago, cannot remember conclusion, but here is the thread:
Debate

Happy Reading /forums/images/graemlins/grin.gif

Jonny

 

[dolphin]

usualy, all of us, used a mercator chart and on this chart the rhumb line is a straight line between two points and the great circle is a shortest distance between two points and on the chart is comming as a oval line with vertex against the pole!
usualy the great circle is used on ocean passage but i think this is more valuable for ships and racers than to cruisers ! if you are sailing in the med for example there shoudl be no difference in miles
mercator projection - imagine the globe into the cylinder and project the earth from the center to the cylinder. after, just open the cylinder and this is the mercator chart ! you will see that the line conecting two point from earth (shortest distance) on the chart will be an oval line (great circle) !

 

[silver-fox]

Yes thats the difference ! But the question is which does your GPS plot over a large distance? Rhumb or Great Circle

 

[demonboy]

Jees...just got halfway through that thread and have had to stop for a breather!

 

[Ariadne]

It could depend the make, model and how you set it up.

Over short distances and travelling from East to West you wouldn't really notice the difference.

Last time I tried this out we were crossing Biscay in a SW'ly direction from Ushant, the GPS worked out the course and distance as a Great Circle route while I plotted it as a straight line on the large scale Mercator chart.

There wasn't a huge difference (not a great distance in the grand sceme of things) but there was a definate difference with the GCR taking more to the west initially.

Worth a play with, if only to confuse those who aren't famillier with Great Circle sailing.

 

[FAITIRA]

Pretty certain it,s great circle.

 

[Channel Ribs]

I agree, most GPSs' know the world is round.

I only say this with confidence because back in the days when I used to do a lot of long haul air travel I liked to see what my pocket gps came up with, it seemed very much that is was never the straight line aproach.

 

[Talbot]

GPS uses an oblate spheroid as its model of the world - cause it is WGS 84, therefore I would be surprised if it converted this back to a mercator projection in order to work out a bearing, thus most likely that the data is a great circle.

 

[BrendanS]

Won't most allow you to choose which you use?

 

[Richard10002]

Using plotter software on my laptop, (S.O.B.), any point to point route is drawn as a straight line on a mercator chart, (flat on the screen, vertical longitude lines, and horizontal latitude lines).

This suggests that it plots a Rhumb Line.

If it was a Great Circle, the line would be an arc on the screen.

Also, AFAIR when checking distance from Lagos to Malta and Crete on the Lowrance Plotter, the route lines are equally straight on the flat screen. Again, AFAIR, the lines of long and lat are vertical and horizontal.

Unless someone proves different, I'm sure that they are Rhumb Lines.

 

[BrendanS]

Garmin website says
Distance
The length (in feet, meters, miles, etc.) between two waypoints or from your current position to a destination waypoint. This length can be measured in straight-line (rhumb line) or great-circle (over the earth) terms. GPS normally uses great circle calculations for distance and desired track.
http://www.garmin.com/aboutGPS/glossary.html

 

[bilbobaggins]

[ QUOTE ]

Can anybody explain to me which course, great circle or rhumb line my GPS would take me on if I was to follow it to a distant destination? (and why!)

[/ QUOTE ]

Professional navigators, during their pro training, mostly take quite a long time to get their heads around all these concepts and definitions. It's incremental - 'brick by brick' - and many of the metaphors used to help 'grasp' the concepts can only go so far.

However, the 'calculator' within the GPS device calculates distance and INITIAL bearing of Point B from Point A, by resolving the bearings/distances of the three sides/angles of a 'spherical triangle'. What's that? It's the triangle on the curved surface of the earth ( or the assumed model of the earth called the spheroid e.g. WGS84 or somesuch ), part of which is your Great Circle course A to B, and the Pole. The 'navigational triangle'......

This is something that traditional navigators had to work with on a daily basis, using spherical trigonometry ( not plane trig 'cos it's about working on a curved surface ). Now the GPS machine does it, once it has worked out where on earth it is.....

By definition, the Great Circle is the shortest distance on the surface of the sphere between A and B. The problem with charts is that they are flat, and the surface they represent is curved, so there have to be distortions - in every different chart projection. You just pick the chart projection - and distortions - you can live with, and for most of us, that's a Mercator.

One of the distortions that appears on a Mercator is that the Great Circle gets bent ON THE PAPER. But the GPS kit still correctly calculates the shortest distance between A and B, and the Initial Bearing to head towards there. It is assumed that once you've been tracking more or less along the Great Circle for a while, you will have the machine recalculate. It then comes up with a new distance and a necessarily slightly-changed new Bearing.

This process gets repeated as often as wanted - once a day for tramp steamer, once an hour for older airliners with live navs on board, once every minute or so for modern airliners feeding into an autopilot......

Some GPS kit can be set - in the options - to work out and present a Rhumb Line course between A and B. OK, if that's what you want...... It can also and usually be set to work from a 'wrong' mapping datum, from Mag instead of True, from a different time-zone, and a whole lot of other 'frigs and tweaks' - few of which are actually useful to guys like us.

In the Northern Hemisphere, the Great Circle route will always 'appear' on a Mercator to be curved towards the nearer Pole, while the Rhumb Line route will 'appear' as a straight line. If you want your Great Circle to 'appear' as a straight line, use a different chart projection such as a Lambert's Conformal.

The Air Force did, for decades.......

Otherwise, don't worry about it. The fastest course is dependent on the winds, not the chart projection. If you've followed that so far, award yourself a pint!


/forums/images/graemlins/smirk.gif

 

[BrendanS]

I'll award myself a pint then! /forums/images/graemlins/smirk.gif

I did an O level in Air Navigation and got an 'A' when I was 16 (so pre gps) and have been working on it ever since /forums/images/graemlins/smile.gif Not to mention using an astro compass from the age of 8. Dad was a pilot in WII, and picked one up cheap, and it fascinated me.

 

[Richard10002]

Whatever Garmin may say, I cant get away from the fact that a C-Map Max chart opened on my laptop has vertical and horizontal long and lat lines, (from Pole to Pole), so it must be based on a Mercator Projection, rather than some type of Gnomonic projection. Route lines in any direction, over long distances, (4k miles, 7k miles), are straight lines, thus Rhumb Lines.

I'm not blowing my own trumpet, but I have a 2nd Class Foreign Going Certificate, (if I could find it), and a Nautical Studies BSc., so unless I was taught b*****ks, or have forgotten what I learned at least twice, (albeit 23 years ago), how can they be Great Circles?

I have no objection to being proved wrong......... <g>

 

[BrendanS]

I guess the only way to find out for that particular package is to read the instructions or call or email them?

 

[pbeard]

Gps is entirely mathematical - it's a great circle.

 

[Salty John]

Re: GPS plots a Rhumb Line or Great Circle?

I agree - GPS always defaults to great circle.

 

[Richard10002]

Re: GPS plots a Rhumb Line or Great Circle?

So, if we plot a route from say, near The turks and Caicos, (20d 52m N 70d 57m W), to the middle of the entrance to the Channel, (49d 11m N 5d 14m W), the rhumb line course and distance are:

062T 3648 Nm

It would be interesting to see what other peoples plotters and GPS suggest as the course and distance for the above route?

ISTR passing fairly close to Bermuda on a Great Circle route.

SOB software says:

"Great Circle, Rhumb Line - Rhumb Line is a straight line course on a Mercator chart (C-MAP charts in SOB use the Mercator Projection). RLs follow a constant magnetic heading, and mathematically are known as a "loxodromic" curve.
Great Circle courses represent the shortest distance between two geographic locations, but require a continuous change in the ship's heading."